Semiconductor device and manufacturing method thereof

ABSTRACT

An object is to suppress conducting-mode failures of a transistor that uses an oxide semiconductor film and has a short channel length. A semiconductor device includes a gate electrode  304,  a gate insulating film  306  formed over the gate electrode, an oxide semiconductor film  308  over the gate insulating film, and a source electrode  310   a  and a drain electrode  310   b  formed over the oxide semiconductor film. The channel length L of the oxide semiconductor film is more than or equal to 1 μm and less than or equal to 50 μm. The oxide semiconductor film has a peak at a rotation angle 2θ in the vicinity of 31° in X-ray diffraction measurement.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/861,587, filed Apr. 12, 2013, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2012-093303 on Apr. 16, 2012, both of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device using an oxidesemiconductor and a manufacturing method thereof. Note that in thisspecification, a semiconductor device refers to a semiconductor elementitself or a device including a semiconductor element. As an example ofsuch a semiconductor element, for example, a transistor (e.g., a thinfilm transistor) can be given. In addition, a semiconductor device alsorefers to a display device such as a liquid crystal display device.

2. Description of the Related Art

A thin film transistor formed over a flat plate such as a glasssubstrate is manufactured using amorphous silicon or polycrystallinesilicon, as typically seen in a liquid crystal display device. A thinfilm transistor manufactured using amorphous silicon has low fieldeffect mobility, but can be formed over a larger glass substrate. Incontrast, a thin film transistor manufactured using crystalline siliconhas high field effect mobility, but due to a crystallization step suchas laser annealing, such a transistor is not always suitable for beingformed over a larger glass substrate.

In view of the foregoing, attention has been drawn to a technique bywhich a thin film transistor is manufactured using an oxidesemiconductor, and such a transistor is applied to an electronic deviceor an optical device. For example, Patent Document 1 and Patent Document2 disclose a technique by which a thin film transistor is manufacturedusing zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as an oxidesemiconductor film and such a transistor is used as a switching elementor the like of an image display device.

However, it is difficult to reduce a channel length of a thin filmtransistor using an amorphous In—Ga—Zn-based oxide semiconductor film(hereinafter referred to as an a-IGZO film) because it is brought into aconducting mode in a region with short channel length. Note that theconducting mode in this specification refers to a mode in which normallyon characteristics or characteristics of a low on/off ratio which causesflow of current are exhibited.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to suppressconducting-mode failures of a transistor that uses an oxidesemiconductor film and has a short channel length.

One embodiment of the present invention is a semiconductor deviceincluding a gate electrode, an oxide semiconductor film, and a sourceelectrode and a drain electrode, in which the length of a channel formedin the oxide semiconductor film is more than or equal to 1 μm and lessthan or equal to 50 μm, and in which the oxide semiconductor film has apeak at a rotation angle 2θ of 31° in X-ray diffraction measurement.

Another embodiment of the present invention is a semiconductor deviceincluding a gate electrode, an oxide semiconductor film, and a sourceelectrode and a drain electrode, in which the length of a channel formedin the oxide semiconductor film is more than or equal to 1 μm and lessthan or equal to 50 μm, and in which the oxide semiconductor film has apeak at a rotation angle 2θ in the vicinity of 31° in X-ray diffractionmeasurement.

In one embodiment of the present invention, it is preferable that thelength of the channel be less than 5 μm.

Further in one embodiment of the present invention, it is preferablethat the oxide semiconductor film have a band gap of more than or equalto 3.1 eV.

Furthermore in one embodiment of the present invention, it is preferablethat the oxide semiconductor film be a film including at least one oxideselected from the group consisting of indium oxide, zinc oxide, galliumoxide, and tin oxide.

Moreover in one embodiment of the present invention, it is preferablethat the oxide semiconductor film be an In—Ga—Zn-based oxidesemiconductor film. Further in one embodiment of the present invention,it is preferable that the oxide semiconductor film include a crystalpart, and that a c-axis of the crystal part be aligned in a directionparallel to a normal vector of a surface on which the oxidesemiconductor film is formed.

Still another embodiment of the present invention is a semiconductordevice including a gate electrode, an oxide semiconductor film, and asource electrode and a drain electrode, in which the length of a channelformed in the oxide semiconductor film is more than or equal to 1 μm andless than or equal to 50 μm, in which the oxide semiconductor filmincludes a crystal part, and in which a c-axis of the crystal part isaligned in a direction parallel to a normal vector of a surface on whichthe oxide semiconductor film is formed. It is preferable that the lengthof the channel be less than 5 μm. Further, it is preferable that theoxide semiconductor film be an In—Ga—Zn-based oxide semiconductor film.

A still further embodiment of the present invention is a method formanufacturing a semiconductor device, which includes the steps offorming a gate electrode over a substrate; forming a gate insulatingfilm over the gate electrode; forming an oxide semiconductor film overthe gate insulating film by heating the substrate and sputtering a metaloxide target under conditions using an oxygen gas and a rare gas;forming an active layer over the gate insulating film by processing theoxide semiconductor film; and forming a source electrode and a drainelectrode over the active layer so that the length of a channel formedin the active layer is more than or equal to 1 μm and less than or equalto 50 μm. In the method, the conditions are conditions where the heatingtemperature of the substrate is more than or equal to 100° C. and theratio of the flow rate of the oxygen gas to the total gas flow is morethan or equal to 70% or conditions where the heating temperature of thesubstrate is more than or equal to 170° C. and the ratio of the flowrate of the oxygen gas to the total gas flow is more than or equal to30%. Further, the oxide semiconductor film has a peak at a rotationangle 2θ in the vicinity of 31° in X-ray diffraction measurement.

In one embodiment of the present invention, it is preferable that themetal oxide target be an In—Ga—Zn-based oxide target.

With one embodiment of the present invention, conducting-mode failuresof a transistor that uses an oxide semiconductor film and has a shortchannel length can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1A is a plan view of a semiconductor device of one embodiment ofthe present invention and FIG. 1B is a cross-sectional view along lineX1-Y1 in FIG. 1A;

FIG. 2A shows results for Sample 1 to Sample 3 and FIG. 2B shows resultsfor Sample 4 to Sample 8 in Example 1;

FIG. 3A shows results for Sample 9 to Sample 13 and FIG. 3B showsresults for Sample 14 to Sample 18 in Example 1;

FIG. 4A shows results for Sample 1 to Sample 3 and FIG. 4B shows resultsfor Sample 4 to Sample 8 in Example 1;

FIG. 5A shows results for Sample 9 to Sample 13 and FIG. 5B showsresults for Sample 14 to Sample 18 in Example 1;

FIGS. 6A to 6C show results of electrical characteristics of transistorswith channel lengths of 2, 3, 4 μm formed under Conditions A in Example2;

FIGS. 7A to 7C show results of electrical characteristics of transistorswith channel lengths of 12, 46, 96 μm formed under Conditions A inExample 2;

FIGS. 8A to 8C show results of electrical characteristics of transistorswith channel lengths of 2, 3, 4 μm formed under Conditions B in Example2;

FIGS. 9A to 9C show results of electrical characteristics of transistorswith channel lengths of 12, 46, 96 μm formed under Conditions B inExample 2;

FIGS. 10A to 10C show results of electrical characteristics oftransistors with channel lengths of 2, 3, 4 μm formed under Conditions Cin Example 2;

FIGS. 11A to 11C show results of electrical characteristics oftransistors with channel lengths of 12, 46, 96 μm formed underConditions C in Example 2;

FIGS. 12A to 12C show results of electrical characteristics oftransistors with channel lengths of 2, 3, 4 μm formed under Conditions Din Example 2; and

FIGS. 13A to 13C show results of electrical characteristics oftransistors with channel lengths of 12, 46, 96 μm formed underConditions D in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will be described below in detailwith reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that the mode and details can bevariously changed without departing from the scope and spirit of thepresent invention. Therefore, the invention should not be construed asbeing limited to the description in the following embodiment.

<CE (Channel-Etched) Structure>

FIG. 1A is a plan view of a semiconductor device of one embodiment ofthe present invention, and FIG. 1B is a cross-sectional view along lineX1-Y1 in FIG. 1A. This semiconductor device includes a transistor havinga bottom-gate structure (also referred to as an inverted staggeredstructure). Note that in FIG. 1A, some components of the transistor(e.g., a gate insulating film 306) are not illustrated for simplicity.

In FIG. 1B, a base insulating film (not shown) is formed over asubstrate 302. It is preferable that a region that spreads from asurface to a depth of 3 nm of the base insulating film have aconcentration of a metal element that is included in the substrate 302of 1×10¹⁸ atoms/cm³ or lower.

A gate electrode 304 is formed over the base insulating film. The gateinsulating film 306 is formed over the base insulating film and the gateelectrode 304. An island-shaped oxide semiconductor film 308 having achannel region is formed over the gate insulating film 306. The oxidesemiconductor film 308 is provided in contact with the gate insulatingfilm 306 in a position that overlaps with the gate electrode 304. Asource electrode 310 a and a drain electrode 310 b are formed over theoxide semiconductor film 308 and the gate insulating film 306. Thesource electrode 310 a and the drain electrode 310 b are electricallyconnected to the oxide semiconductor film 308.

The oxide semiconductor film 308 has a channel length of more than orequal to 1 μm and less than or equal to 50 μm (preferably less than 5μm) and a channel width W (see FIG. 1A). Further, it is preferable thatthe oxide semiconductor film 308 have a peak at a rotation angle 2θ inthe vicinity of 31° in X-ray diffraction measurement and the band gap ofthe oxide semiconductor film 308 be more than or equal to 3.1 eV. Thedetails of the oxide semiconductor film 308 are described later.

Further, an interlayer insulating film 312 and a planarizationinsulating film 314 may be provided over the transistor. In detail, theinterlayer insulating film 312 may be provided over the oxidesemiconductor film 308, the source electrode 310 a, and the drainelectrode 310 b, and the planarization insulating film 314 may beprovided over the interlayer insulating film 312.

With this embodiment using the oxide semiconductor film 308 having apeak at a rotation angle 2θ in the vicinity of 31° in X-ray diffractionmeasurement and having a band gap of more than or equal to 3.1 eV, thetransistor that uses the oxide semiconductor film and has a shortchannel length can have suppressed conducting-mode failures. In thisspecification and the like, having a peak at a rotation angle 2θ in thevicinity of 31° means having a peak at a rotation angle 2θ of 31° withan error of plus or minus 1°.

[Detailed Description of Oxide Semiconductor Film]

The oxide semiconductor film 308 is preferably a CAAC-OS (c-axis alignedcrystalline oxide semiconductor) film.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal parts are includedin an amorphous phase. Note that in most cases, the crystal part fitsinside a cube whose one side is less than 100 nm. From an observationimage obtained with a transmission electron microscope (TEM), a boundarybetween an amorphous part and a crystal part in the CAAC-OS film is notclear. Further, with the TEM, a grain boundary is not found in theCAAC-OS film. Thus, in the CAAC-OS film, a reduction in electronmobility due to the grain boundary is suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface on whichthe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic arrangement which is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that, among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. In this specification andthe like, a simple term “perpendicular” includes a range from 85° to95°. In addition, a simple term “parallel” includes a range from −5° to5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher in some cases.Further, when an impurity is added to the CAAC-OS film, the crystal partin a region to which the impurity is added becomes amorphous in somecases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface onwhich the CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface on which the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film).

Note that the c-axes of the crystal parts are aligned in the directionparallel to a normal vector of the surface on which the CAAC-OS film isformed or a normal vector of the surface of the CAAC-OS film. Thecrystal parts are formed by film formation or by performing treatmentfor crystallization such as heat treatment after film formation.

With use of the CAAC-OS film in a transistor, change in electricalcharacteristics of the transistor due to irradiation with visible lightor ultraviolet light can be reduced. Change and variation in thresholdvoltage can be suppressed. Thus, the transistor has high reliability.

In a crystal part or a crystalline oxide semiconductor film, defects inthe bulk can be further reduced. Further, when the surface flatness ofthe crystal part or the crystalline oxide semiconductor film isenhanced, a transistor including the oxide semiconductor film can havehigher field-effect mobility than a transistor including an amorphousoxide semiconductor film. In order to improve the surface flatness ofthe oxide semiconductor film, the oxide semiconductor film is preferablyformed over a flat surface. Specifically, the oxide semiconductor ispreferably formed over a surface with an average surface roughness (Ra)of less than or equal to 0.15 nm, preferably less than or equal to 0.1nm.

Note that the average surface roughness Ra is obtained by expandingcenter line average surface roughness that is defined by JIS B 0601 intothree dimensions for application to a surface, and Ra can be expressedas the average value of the absolute values of deviations from areference surface to a specific surface and is defined by Formula 1.

$\begin{matrix}{{Ra} = {\frac{1}{S_{0}}{\int_{y_{2}}^{y_{1}}{\int_{x_{2}}^{x_{1}}{{{{f\left( {x,y} \right)} - Z_{0}}}\ {x}\ {y}}}}}} & \left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In the above formula, S₀ represents the area of a measurement surface (arectangular region which is defined by four points at coordinates (x₁,y₁), (x₁, y₂), (x₂, y₁), and (x₂, y₂)), and Z₀ represents the averageheight of the measurement surface. Ra can be measured using an atomicforce microscope (AFM).

For the oxide semiconductor film, an oxide semiconductor having a widerband gap than that of silicon, i.e., 1.1 eV, is preferably used. Forexample, an In—Ga—Zn-based oxide having a band gap of 3.15 eV, an indiumoxide having a band gap of about 3.0 eV, an indium tin oxide having aband gap of about 3.0 eV, an indium gallium oxide having a band gap ofabout 3.3 eV, an indium zinc oxide having a band gap of about 2.7 eV, atin oxide having a band gap of about 3.3 eV a zinc oxide having a bandgap of about 3.37 eV, or the like can be preferably used. With the useof such a material, the off-state current of the transistor can be keptextremely low. Note that in one embodiment of the present invention, theband gap of the oxide semiconductor film is preferably more than orequal to 3.1 eV.

An oxide semiconductor used for the oxide semiconductor film preferablyincludes at least one selected from the group consisting of indium (In),zinc (Zn), and gallium (Ga). In particular, In and Zn are preferablyincluded. As a stabilizer for reducing a variation in electricalcharacteristics among transistors including the oxide semiconductor, tin(Sn) is preferably included.

For example, as the oxide semiconductor, any of the following can beused: indium oxide; tin oxide; zinc oxide; a two-component metal oxidesuch as an In—Zn-based oxide, a Sn—Zn-based oxide, or In—Ga-based oxide;a three-component metal oxide such as an In—Ga—Zn-based oxide (alsoreferred to as IGZO), In—Sn—Zn-based oxide, Sn—Ga—Zn-based oxide; and afour-component metal oxide such as an In—Sn—Ga—Zn-based oxide.

Here, an “In—Ga—Zn-based oxide” means an oxide including In, Ga, and Znas its main components and there is no particular limitation on theratio of In:Ga:Zn. The In—Ga—Zn-based oxide may include a metal elementother than the In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0, m is notan integer) may be used as an oxide semiconductor. Note that Mrepresents one or more metal elements selected from Ga, Fe, Mn, and Co,or the above-described element as a stabilizer. Alternatively, as theoxide semiconductor, a material represented by In₂SnO₅(ZnO)_(n) (n>0, nis an integer) may be used.

For example, an In—Ga—Zn-based oxide with an atomic ratio whereIn:Ga:Zn=1:1:1, In:Ga:Zn=3:1:2, or In:Ga:Zn=2:1:3, or an oxide whosecomposition is in the neighborhood of the above compositions can beused.

In a formation step of the oxide semiconductor film, it is preferablethat hydrogen or water be contained in the oxide semiconductor film aslittle as possible. For example, it is preferable that the substrate bepreheated in a preheating chamber of a sputtering apparatus aspretreatment for formation of the oxide semiconductor film so that animpurity such as hydrogen or moisture adsorbed to the substrate areeliminated and removed. Then, the oxide semiconductor film is preferablyformed in a film formation chamber from which remaining moisture isremoved.

In order to remove the moisture in the preheating chamber and the filmformation chamber, an entrapment vacuum pump, for example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. Further,an evacuation unit may be a turbo pump provided with a cold trap. Fromthe preheating chamber and the film formation chamber which areevacuated with a cryopump, a hydrogen atom, a compound containing ahydrogen atom such as water (H₂O) (preferably, also a compoundcontaining a carbon atom), and the like are removed, whereby theconcentration of an impurity such as hydrogen or moisture in the oxidesemiconductor film can be reduced.

Note that an In—Ga—Zn-based oxide film is formed as the oxidesemiconductor film by a sputtering method. The oxide semiconductor filmcan be formed by a sputtering method in a rare gas (typically argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gasand oxygen.

As a target used for forming an In—Ga—Zn-based oxide film as the oxidesemiconductor film by a sputtering method, for example, a metal oxidetarget with an atomic ratio where In:Ga:Zn=1:1:1, a metal oxide targetwith an atomic ratio where In:Ga:Zn=3:1:2, or a metal oxide target withan atomic ratio where In:Ga:Zn=2:1:3 can be used. However, a materialand composition of a target used for formation of the oxidesemiconductor film are not limited to the above.

Further, when the oxide semiconductor film is formed using the abovemetal oxide target, the composition of the film formed over thesubstrate is different from the composition of the target in some cases.For example, when the metal oxide target of In:Ga:Zn=1:1:1 [atomicratio] is used, the composition of the oxide semiconductor film, whichis a thin film, becomes In:Ga:Zn=1:1:0.6 to 1:1:0.8 [atomic ratio] insome cases, although it depends on the film formation conditions. Thisis because in formation of the oxide semiconductor film, Zn is sublimed,or because the sputtering rate differs between the components of In, Ga,and Zn.

Accordingly, in order to form a thin film having a desired compositionratio, the composition of the metal oxide target needs to be adjusted inadvance. For example, in order to make the composition of the oxidesemiconductor film, which is a thin film, be In:Ga:Zn=1:1:1 [atomicratio], the composition of the metal oxide target is preferablyIn:Ga:Zn=1:1:1.5 [atomic ratio]. In other words, the percentage of Zncontent in the metal oxide target is preferably made higher in advance.The composition of the target is not limited to the above value, and canbe adjusted as appropriate depending on the film formation conditions orthe composition of the thin film to be formed. Further, it is preferableto increase the percentage of Zn content in the metal oxide targetbecause the obtained thin film can have higher crystallinity.

The relative density of the metal oxide target with respect to a singlecrystal consisting of the same material as the metal oxide target ismore than or equal to 90% and less than or equal to 100%, preferablymore than or equal to 95% and less than or equal to 99.9%. By using themetal oxide target with high relative density with respect to a singlecrystal consisting of the same material as the metal oxide target, adense oxide semiconductor film can be formed.

As a sputtering gas used for forming the oxide semiconductor film, it ispreferable to use a high-purity gas from which impurities such ashydrogen, water, hydroxyl groups, or hydrides are removed.

There are three methods for forming a CAAC-OS film when the CAAC-OS filmis used as the oxide semiconductor film. The first method is to form anoxide semiconductor film at a temperature higher than or equal to 100°C. and lower than or equal to 450° C., whereby crystal parts in whichthe c-axes are aligned in the direction parallel to a normal vector of asurface on which the oxide semiconductor film is formed or a normalvector of a surface of the oxide semiconductor film are formed in theoxide semiconductor film. The second method is to form an oxidesemiconductor film with a small thickness and then heat it at atemperature higher than or equal to 200° C. and lower than or equal to700° C., whereby crystal parts in which the c-axes are aligned in thedirection parallel to a normal vector of a surface on which the oxidesemiconductor film is formed or a normal vector of a surface of theoxide semiconductor film are formed in the oxide semiconductor film. Thethird method is to form a first oxide semiconductor film with a smallthickness, then heat it at a temperature higher than or equal to 200° C.and lower than or equal to 700° C., and form a second oxidesemiconductor film, whereby crystal parts in which the c-axes arealigned in the direction parallel to a normal vector of a surface onwhich the oxide semiconductor film is formed or a normal vector of asurface of the oxide semiconductor film are formed in the second oxidesemiconductor film.

By heating the substrate during film formation, the concentration of animpurity such as hydrogen or water in the formed oxide semiconductorfilm can be reduced. In addition, damage by sputtering can be reduced,which is preferable. The oxide semiconductor film may be formed by anALD (atomic layer deposition) method, an evaporation method, a coatingmethod, or the like.

Note that when a crystalline (single-crystal or microcrystalline) oxidesemiconductor film other than a CAAC-OS film is formed as the oxidesemiconductor film, the film formation temperature is not particularlylimited.

As a method for processing the oxide semiconductor film, a wet etchingmethod or a dry etching method can be used to etch the oxidesemiconductor film. An etching gas such as BCl₃, Cl₂, or O₂ can be usedin the dry etching method. Further, a dry etching apparatus using ahigh-density plasma source such as electron cyclotron resonance (ECR) orinductive coupled plasma (ICP) can be used to improve a dry etchingrate.

After the oxide semiconductor film is formed, the oxide semiconductorfilm may be subjected to heat treatment. The temperature of the heattreatment is higher than or equal to 300° C. and lower than or equal to700° C., or lower than the strain point of the substrate. Through theheat treatment, excess hydrogen (including water and a hydroxyl group)contained in the oxide semiconductor film can be removed. Note that theheat treatment is also referred to as dehydration treatment(dehydrogenation treatment) in this specification and the like in somecases.

The heat treatment can be performed in such a manner that, for example,an object is introduced into an electric furnace in which a resistanceheater or the like is used and heated at 450° C. in a nitrogenatmosphere for an hour. The oxide semiconductor film is not exposed tothe air during the heat treatment so that entry of water or hydrogen canbe prevented.

Note that a heat treatment apparatus is not limited to an electricfurnace, and may be a device for heating an object by heat conduction orheat radiation from a medium such as a heated gas. For example, an RTA(rapid thermal anneal) apparatus such as a GRTA (gas rapid thermalanneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus canbe used. An LRTA apparatus is an apparatus for heating an object byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for performing heat treatment using ahigh-temperature gas. As the gas, an inert gas which does not react withan object by heat treatment, such as nitrogen or a rare gas such asargon is used.

For example, as the heat treatment, the GRTA process may be performed asfollows. The object is put in a heated inert gas atmosphere, heated forseveral minutes, and taken out of the inert gas atmosphere. The GRTAprocess enables high-temperature heat treatment in a short time.Moreover, the GRTA process can be employed even when the temperatureexceeds the upper temperature limit of the object. Note that the inertgas may be switched to a gas including oxygen during the process.

Note that as the inert gas atmosphere, an atmosphere that containsnitrogen or a rare gas (e.g., helium, neon, or argon) as its maincomponent and does not contain water, hydrogen, or the like ispreferably used. For example, the purity of nitrogen or a rare gas suchas helium, neon, or argon introduced into a heat treatment apparatus isgreater than or equal to 6 N (99.9999%), preferably greater than orequal to 7 N (99.99999%) (that is, the concentration of the impuritiesis less than or equal to 1 ppm, preferably less than or equal to 0.1ppm).

The dehydration treatment (dehydrogenation treatment) might beaccompanied by elimination of oxygen which is a main constituentmaterial for an oxide semiconductor film, leading to a reduction inoxygen. An oxygen vacancy exists in a portion where oxygen is eliminatedin an oxide semiconductor film, and a donor level which leads to achange in the electrical characteristics of a transistor is formed owingto the oxygen vacancy. Therefore, in the case where the dehydrationtreatment (dehydrogenation treatment) is performed, oxygen is preferablysupplied to the oxide semiconductor film. By supply of oxygen to theoxide semiconductor film, an oxygen vacancy in the film can be filled.

The oxygen vacancy in the oxide semiconductor film may be filled in thefollowing manner for example: after the oxide semiconductor film issubjected to the dehydration treatment (dehydrogenation treatment), ahigh-purity oxygen gas, a nitrous oxide gas, a high-purity nitrous oxidegas, or ultra dry air (the moisture amount is less than or equal to 20ppm (−55° C. by conversion into a dew point), preferably less than orequal to 1 ppm, further preferably less than or equal to 10 ppb, in themeasurement with the use of a dew point meter of a cavity ring downlaser spectroscopy (CRDS) system) is introduced into the same furnace.It is preferable that water, hydrogen, and the like be not contained inthe oxygen gas or the nitrous oxide gas. The purity of the oxygen gas orthe nitrous oxide gas which is introduced into the heat treatmentapparatus is preferably 6N (99.9999%) or more, further preferably 7N(99.99999%) or more (i.e., the impurity concentration in the oxygen gasor the nitrous oxide gas is preferably less than or equal to 1 ppm,further preferably less than or equal to 0.1 ppm).

As an example of a method of supplying oxygen to the oxide semiconductorfilm, oxygen (including at least any one of oxygen radicals, oxygenatoms, and oxygen ions) may be added to the oxide semiconductor film. Anion implantation method, an ion doping method, a plasma immersion ionimplantation method, plasma treatment, or the like can be used as amethod for adding oxygen.

As another example of a method for supplying oxygen to the oxidesemiconductor film, the base insulating film, the gate insulating filmto be formed later, or the like may be heated to release part of oxygenand supply oxygen to the oxide semiconductor film.

As described above, after formation of the oxide semiconductor film, itis preferable that dehydration treatment (dehydrogenation treatment) beperformed to remove hydrogen or moisture from the oxide semiconductorfilm so that the oxide semiconductor film is highly purified to containas few impurities as possible, and that oxygen be added to the oxidesemiconductor in which oxygen is reduced by the dehydration treatment(dehydrogenation treatment) or excess oxygen is supplied to fill oxygenvacancies in the oxide semiconductor film. Supplying oxygen to an oxidesemiconductor film may be expressed as oxygen adding treatment ortreatment for making an oxygen-excess state.

In this manner, hydrogen or moisture is removed from the oxidesemiconductor film by dehydration treatment (dehydrogenation treatment)and an oxygen vacancy therein is filled by oxygen adding treatment,whereby the oxide semiconductor film can be turned into an electricallyi-type (intrinsic) or substantially i-type oxide semiconductor film.Specifically, the concentration of hydrogen in the oxide semiconductorfilm is lower than or equal to 5×10¹⁹ atoms/cm³, preferably lower thanor equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to5×10¹⁷ atoms/cm³. Note that the concentration of hydrogen in the oxidesemiconductor film is measured by secondary ion mass spectrometry(SIMS).

The number of carriers generated due to a donor in the oxidesemiconductor film, in which hydrogen concentration is reduced to asufficiently low concentration so that the oxide semiconductor film ispurified and in which defect states in an energy gap due to oxygendeficiency are reduced by sufficiently supplying oxygen as describedabove, is very small (close to zero); the carrier concentration in theoxide semiconductor film is less than 1×10¹²/cm³, preferably less than1×10¹¹/cm³, further preferably less than 1.45×10¹⁰/cm³. In a transistorincluding such an oxide semiconductor film, the off-state current (perunit channel width (1 μm) here) at room temperature (25° C.), forexample, is less than or equal to 100 zA (1 zA (zeptoampere) is 1×10⁻²¹A), preferably less than or equal to 10 zA, further preferably less thanor equal to 100 yA (1 yA (yoctoampere) is 1×10⁻²⁴ A). The transistorwith very excellent off-state current characteristics can be obtainedwith the use of such an i-type (intrinsic) or substantially i-type oxidesemiconductor.

<Method for Manufacturing Transistor>

Next, a method for manufacturing the semiconductor device illustrated inFIG. 1B is described. First, a base insulating film (not shown) isformed over the substrate 302.

As the substrate 302, a substrate of a glass material such asaluminosilicate glass, aluminoborosilicate glass, barium borosilicateglass, or the like is used. In terms of mass production, a mother glasswith the following size is preferably used for the substrate 302: the8th generation (2160 mm×2460 mm); the 9th generation (2400 mm×2800 mm,or 2450 mm×3050 mm); the 10th generation (2950 mm×3400 mm); or the like.A mother glass considerably shrinks when the treatment temperature ishigh and the treatment time is long. Thus, in the case where massproduction is performed with the use of the mother glass, the heatingtemperature in the manufacturing process is preferably 600° C. or lower,further preferably 450° C. or lower, still further preferably 350° C. orlower.

As the base insulating film, a film of silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, orthe like can be used. The aluminum oxide film is formed by a sputteringmethod and preferably has a density of 3.2 g/cm³ or more, furtherpreferably 3.6 g/cm³ or more. With the use of the above-describedaluminum oxide film as the base insulating film, an impurity can beprevented from being diffused from the substrate 302 into thetransistor. The impurity from the substrate 302 is, for example,hydrogen, a metal element, or the like. As the metal element, elementssuch as sodium, aluminum, magnesium, calcium, strontium, barium,silicon, and boron can be given. The base insulating film can be formedto a thickness of more than or equal to 5 nm and less than or equal to150 nm (preferably more than or equal to 10 nm and less than or equal to100 nm).

In a region that spreads from a surface to a depth of 3 nm of the baseinsulating film, the concentration of a metal element that is includedin the glass substrate is preferably 1×10¹⁸ atoms/cm³ or lower.

The base insulating film is preferably a film which releases a smallamount of water (H₂O) or hydrogen (H₂). For example, an aluminum oxidefilm can be used as the base insulating film. It is preferable that theamount of water (H₂O) released from the aluminum oxide film be less thanor equal to 5×10¹⁵ atoms/cm³, further preferably less than or equal to1×10¹⁵ atoms/cm³. Further, it is preferable that the amount of hydrogen(H₂) released from the aluminum oxide film be less than or equal to5×10¹⁵ atoms/cm³, further preferably less than or equal to 1×10¹⁵atoms/cm³.

For example, in the case where a film which releases a large amount ofhydrogen or water is used as the base insulating film, there is apossibility that water or hydrogen is released from the base insulatingfilm in a process of forming the transistor and diffused into the oxidesemiconductor film 308 in the transistor. By using the base insulatingfilm which releases the above-described amount of hydrogen or water, animpurity diffused into the transistor can be reduced, which leads to ahighly reliable semiconductor device.

Note that the amount of released water and the amount of releasedhydrogen can be measured by thermal desorption spectroscopy (TDS).

Next, after a conductive film is formed over the base insulating film,the gate electrode 304 is formed by a photolithography step and anetching step, and then, the gate insulating film 306 is formed over thebase insulating film and the gate electrode 304.

The gate electrode 304 can be formed by a sputtering method or the liketo have a single-layer structure or a stacked-layer structure using ametal material such as molybdenum, titanium, tantalum, tungsten,aluminum, copper, neodymium, or scandium, or an alloy materialcontaining at least any one of these materials.

The gate insulating film 306 can be formed by, for example, a PE-CVDmethod or the like, using silicon oxide, gallium oxide, aluminum oxide,silicon nitride, silicon oxynitride, aluminum oxynitride, siliconnitride oxide, or the like. The thickness of the gate insulating film306 can be, for example, more than or equal to 10 nm and less than orequal to 500 nm, preferably more than or equal to 50 nm and less than orequal to 300 nm. A film that can prevent diffusion of an impurity fromthe substrate 302 can be used as the gate insulating film 306; in such acase, the base insulating film can be omitted.

It is preferable that the gate insulating film 306 include oxygen in aportion which is in contact with the oxide semiconductor film 308 to beformed later. In particular, the gate insulating film 306 preferablyincludes oxygen in an amount which exceeds at least the stoichiometriccomposition. For example, in the case where silicon oxide is used forthe gate insulating film 306, the composition formula is preferablySiO_(2+a) (a>0). In this embodiment, silicon oxide of SiO_(2+a) (a>0) isused for the gate insulating film 306. By using this silicon oxide forthe gate insulating film 306, oxygen can be supplied to the oxidesemiconductor film 308 to be formed later and thus the oxidesemiconductor film 308 can have excellent electrical characteristics.

The gate insulating film 306 can be formed using a high-k material suchas hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0,y>0)), hafnium silicate to which nitrogen is added (HfSiO_(x)N_(y) (x>0,y>0)), hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), or lanthanum oxide.By using such a material, gate leakage current can be reduced. Further,the gate insulating film 306 may have either a single-layer structure ora stacked-layer structure.

Next, heat treatment may be performed on the substrate 302 provided withthe gate insulating film 306.

The heat treatment can be performed using, for example, an electricfurnace or an apparatus for heating an object by heat conduction or heatradiation from a heating element such as a resistance heating element. Arapid thermal anneal (RTA) apparatus such as a gas rapid thermal anneal(GRTA) apparatus or a lamp rapid thermal anneal (LRTA) apparatus can beused. An LRTA apparatus is an apparatus for heating an object byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the high-temperature gas, an inert gas whichdoes not react with an object by heat treatment, such as nitrogen or arare gas like argon, is used. Alternatively, oxygen may be used asanother high-temperature gas. When oxygen is used, release of oxygenfrom the gate insulating film 306 can be inhibited or supply of oxygento the gate insulating film 306 can be performed.

In the case where the mother glass is used as the substrate 302, becausehigh treatment temperature and long treatment time considerably shrinkthe mother glass, the treatment temperature of the heat treatment ispreferably higher than or equal to 200° C. and lower than or equal to450° C., further preferably higher than or equal to 250° C. and lowerthan or equal to 350° C.

An impurity such as water or hydrogen in the gate insulating film 306can be removed by the heat treatment. Further, by the heat treatment,the defect density in the gate insulating film 306 can be reduced. Thereduction of the impurity in the gate insulating film 306 or the defectdensity in the film leads to improvement in reliability of thesemiconductor device. For example, degradation of the semiconductordevice by a negative bias stress test with light irradiation, which isone of the reliability tests for semiconductor devices, can besuppressed.

The heat treatment may be performed as pretreatment for formation of theoxide semiconductor film 308 to be formed later. For example, after thegate insulating film 306 is formed, heat treatment may be performed in avacuum in a preheating chamber of a sputtering apparatus and the oxidesemiconductor film 308 may then be formed.

Furthermore, the heat treatment may be performed more than once. Forexample, after the gate insulating film 306 is formed, heat treatmentmay be performed in a nitrogen atmosphere with an electric furnace orthe like, then heat treatment may be performed in a vacuum in apreheating chamber of a sputtering apparatus, and then the oxidesemiconductor film 308 may then be formed.

Next, an oxide semiconductor film is formed over the gate insulatingfilm 306, and a photolithography step and an etching step are performed;thus, the island-shaped semiconductor film 308 is formed.

The oxide semiconductor film 308 and a manufacturing method thereof aredescribed in detail in the section [Detailed Description of OxideSemiconductor Film]. In one embodiment of the present invention, theoxide semiconductor film 308 serving as an active layer over the gateinsulating film 306 is preferably formed in such a manner that anIn—Ga—Zn-based oxide target is sputtered under first and secondconditions using an oxygen gas and a rare gas while the substrate 302 isheated to form an oxide semiconductor film over the gate insulating film306, and the oxide semiconductor film is processed. The first conditionsare preferably conditions where the heating temperature of the substrate302 is more than or equal to 100° C. and the ratio of the flow rate ofthe oxygen gas to the total gas flow is more than or equal to 70%. Thesecond conditions are preferably conditions where the heatingtemperature of the substrate 302 is more than or equal to 170° C. andthe ratio of the flow rate of the oxygen gas to the total gas flow ismore than or equal to 30%. In this manner, the oxide semiconductor film308 having a peak at a rotation angle 2θ in the vicinity of 31° in X-raydiffraction measurement and having a band gap of more than or equal to3.1 eV can be formed.

Next, a conductive film is formed over the gate insulating film 306 andthe oxide semiconductor film 308 and is subjected to a photolithographystep and an etching step, whereby the source electrode 310 a and thedrain electrode 310 b which are electrically connected to the oxidesemiconductor film 308 are formed. The channel length L of the oxidesemiconductor film 308 is more than or equal to 1 μm and less than orequal to 50 μm, preferably less than 5 μm. At this stage, the transistoris formed.

A conductive film used for the source electrode 310 a and the drainelectrode 310 b is fanned using a metal film including an elementselected from Al, Cr, Cu, Ta, Ti, Mo, and W, or a metal nitride filmincluding any of the above elements as a component (a titanium nitridefilm, a molybdenum nitride film, or a tungsten nitride film) can beused. Alternatively, a structure in which a film of a high-melting-pointmetal such as Ti, Mo, or W or a metal nitride film thereof (a titaniumnitride film, a molybdenum nitride film, or a tungsten nitride film) isfanned over or/and below a metal film such as an Al film or a Cu film,may be employed.

Next, the interlayer insulating film 312 and the planarizationinsulating film 314 are formed over the transistor. The interlayerinsulating film 312 can be formed using the same material and method asthe gate insulating film 306.

As the planarization insulating film 314, for example, an organic resinmaterial such as a polyimide-based resin, an acrylic-based resin, or abenzocyclobutene-based resin can be used. With the planarizationinsulating film 314, the surface roughness of the transistor can bereduced.

Further, a conductive film (not shown) may be formed over theplanarization insulating film 314. For the conductive film, a conductivematerial with a light-transmitting property such as indium oxide-tinoxide (ITO: indium tin oxide) or indium oxide-zinc oxide can be used.Note that the material of the conductive film is not limited to theabove. For example, a metal film (a film of aluminum, titanium, or thelike) may be used. Such a metal film is preferably used because thetransistor can be shielded from external light.

The conductive film also has a function of shielding the transistor fromstatic charges (what is called an electrostatic discharge: ESD). Withthe conductive film over the transistor, charge due to electrostaticdischarge or the like can be dissipated.

Through the above-described manufacturing steps, the semiconductordevice illustrated in FIG. 1B can be manufactured.

The back channel side in the channel-etched structure is exposed toplasma treatment at the time of channel etching. In the case wherevacancies are generated in the active layer, long channel length L makesit difficult to generate conducting-mode failures, and short channellength L makes it easy to generate conducting-mode failures. However, inthe channel-etched structure in one embodiment of the present invention,the oxide semiconductor film 308 formed using IGZO that has a peak at arotation angle 2θ in the vicinity of 31° in X-ray diffractionmeasurement and has a band gap of more than or equal to 3.1 eV is used.The use of the oxide semiconductor film 308 can decrease the probabilityof generating vacancies in the oxide semiconductor film 308, therebysuppressing conducting-mode failures.

As described above, even the transistor that uses an oxide semiconductorfilm and has a short channel length L can have suppressedconducting-mode failures. Accordingly, manufacture of a transistor withshort channel length is possible; the channel length can be less than 5μm, or less than or equal to 2 μm. Consequently, transistors with a widerange of channel length/channel width (L/W) can be manufactured.Further, since the channel length can be shortened, the size of thetransistor can be reduced. This is advantageous for improvements inaperture ratio and definition in the case of using the transistor in apanel, or can reduce a driver region to reduce the frame size in thecase of using the transistor in a driver.

EXAMPLE 1

In Example 1, oxide semiconductor films including indium, gallium, andzinc (expressed as IGZO films below) were formed, and the band gap (alsoreferred to as energy gap and expressed as Eg below) and thecrystallinity of each of the IGZO films were evaluated. Note that in Egevaluation, values measured by spectroscopic ellipsometry wereevaluated, and in crystallinity evaluation, measurement was performedusing X-ray diffraction (XRD) (this measurement is expressed as XRDmeasurement below).

The IGZO films (Sample 1 to Sample 18) were formed using a metal oxidetarget with an atomic ratio of In:Ga:Zn=1:1:1 under fixed conditionswhere the film formation pressure was 0.6 Pa, the film formation powerwas 5 kw, and the film thickness was 100 nm, at various substratetemperatures and various O₂ flow rates (various ratios of O₂ flow rate).The details of the conditions for Sample 1 to Sample 18 are describedbelow.

(Sample 1)

-   Substrate temperature=room temperature (R.T.), O₂ flow rate=200 sccm    (ratio of O₂ flow rate=100%)

(Sample 2)

-   Substrate temperature=room temperature (R.T.), Ar/O₂ flow    rates=100/100 sccm (ratio of O₂ flow rate=50%)

(Sample 3)

-   Substrate temperature=room temperature (R.T.), Ar/O₂ flow    rates=180/20 sccm (ratio of O₂ flow rate=10%)

(Sample 4)

-   Substrate temperature=100° C., O₂ flow rate=200 sccm (ratio of O₂    flow rate=100%)

(Sample 5)

-   Substrate temperature=100° C., Ar/O₂ flow rates=60/140 sccm (ratio    of O₂ flow rate=70%)

(Sample 6)

-   Substrate temperature=100° C., Ar/O₂ flow rates=100/100 sccm (ratio    of O₂ flow rate=50%)

(Sample 7)

-   Substrate temperature=100° C., Ar/O₂ flow rates=140/60 sccm (ratio    of O₂ flow rate=30%)

(Sample 8)

-   Substrate temperature=100° C., Ar/O₂ flow rates=180/20 sccm (ratio    of O₂ flow rate=10%)

(Sample 9)

-   Substrate temperature=170° C., O₂ flow rate=200 sccm (ratio of O₂    flow rate=100%)

(Sample 10)

-   Substrate temperature=170° C., Ar/O₂ flow rates=60/140 sccm (ratio    of O₂ flow rate=70%)

(Sample 11)

-   Substrate temperature=170° C., Ar/O₂ flow rates=100/100 sccm (ratio    of O₂ flow rate=50%)

(Sample 12)

-   Substrate temperature=170° C., Ar/O₂ flow rates=140/60 seem (ratio    of O₂ flow rate=30%)

(Sample 13)

-   Substrate temperature=170° C., Ar/O₂ flow rates=180/20 sccm (ratio    of O₂ flow rate=10%)

(Sample 14)

-   Substrate temperature=200° C., O₂ flow rate=200 sccm (ratio of O₂    flow rate=100%)

(Sample 15)

-   Substrate temperature=200° C., Ar/O₂ flow rates=60/140 sccm (ratio    of O₂ flow rate=70%)

(Sample 16)

-   Substrate temperature=200° C., Ar/O₂ flow rates=100/100 sccm (ratio    of O₂ flow rate=50%)

(Sample 17)

-   Substrate temperature=200° C., Ar/O₂ flow rates=140/60 sccm (ratio    of O₂ flow rate=30%)

(Sample 18)

-   Substrate temperature=200° C., Ar/O₂ flow rates=180/20 sccm (ratio    of O₂ flow rate=10%)

First, the results of the Eg measurement by spectroscopic ellipsometryare shown in FIGS. 2A and 2B and FIGS. 3A and 3B. FIG. 2A shows theresults for Sample 1 to Sample 3, FIG. 2B shows the results for Sample 4to Sample 8, FIG. 3A shows the results for Sample 9 to Sample 13, andFIG. 3B shows the results for Sample 14 to 18. In FIGS. 2A and 2B andFIGS. 3A and 3B, the vertical axis represents Eg (eV), and thehorizontal axis represents the ratio of O₂ flow rate (%).

FIG. 2A indicates that under the condition where the substratetemperature is room temperature (R.T.), Eg is almost constant at around3.06 eV with varying ratio of O₂ flow rate. Further, FIG. 2B indicatesthat under the condition where the substrate temperature is 100° C., Egincreases in accordance with an increase in the ratio of O₂ flow rateand that Sample 5 with a ratio of O₂ flow rate of 70% and Sample 4 witha ratio of O₂ flow rate of 100% have an Eg value of more than 3.10 eV.Furthermore, FIG. 3A indicates that under the condition where thesubstrate temperature is 170° C., Eg increases in accordance with anincrease in the ratio of O₂ flow rate and that Sample 12 with a ratio ofO₂ flow rate of 30%, Sample 11 with a ratio of O₂ flow rate of 50%,Sample 10 with a ratio of O₂ flow rate of 70%, and Sample 9 with a ratioof O₂ flow rate of 100% have an Eg value of more than 3.10 eV. Further,FIG. 3B indicates that under the condition where the substratetemperature is 200° C., Eg increases in accordance with an increase inthe ratio of O₂ flow rate and that Sample 17 with a ratio of O₂ flowrate of 30%, Sample 16 with a ratio of O₂ flow rate of 50%, Sample 15with a ratio of O₂ flow rate of 70%, and Sample 14 with a ratio of O₂flow rate of 100% have an Eg value of more than 3.10 eV.

The above results show that Eg of the IGZO film can be controlled bychanging the substrate temperature and the ratio of O₂ flow rate, whichare film formation conditions of the IGZO film.

Next, the results of XRD measurement are shown in FIGS. 4A and 4B andFIGS. 5A and 5B. FIG. 4A shows the results for Samples 1 to Sample 3,FIG. 4B shows the results for Sample 4 to Sample 8, FIG. 5A shows theresults for Sample 9 to Sample 13, and FIG. 5B shows the results forSample 14 to Sample 18. In FIGS. 4A and 4B and FIGS. 5A and 5B, thevertical axis represents the intensity of X-ray diffraction (arbitraryunit), and the horizontal axis represents the rotation angle 2θ (deg.).

In FIG. 4A, no peak that indicates crystallinity is observed for Sample1 to Sample 3 with a substrate temperature of room temperature (R.T.).In FIG. 4B, no peak that indicates crystallinity is observed for Sample8, Sample 7, and Sample 6 with a substrate temperature of 100° C. andratios of O₂ flow rate of 10%, 30%, and 50%. Meanwhile for Sample 5 andSample 4 with a substrate temperature of 100° C. and ratios of O₂ flowrate of 70% and 100%, peaks indicating crystallinity are observed at 2θof in the vicinity of 31°. Further in FIG. 5A, no peak that indicatescrystallinity is observed for Sample 13 with a substrate temperature of170° C. and a ratio of O₂ flow rate of 10%. Meanwhile for Sample 12,Sample 11, Sample 10, and Sample 9 with a substrate temperature of 170°C. and ratios of O₂ flow rate of 30%, 50%, 70%, and 100%, peaksindicating crystallinity are observed at 2θ of in the vicinity of 31°.Furthermore, in FIG. 5B, no peak that indicates crystallinity isobserved for Sample 18 with a substrate temperature of 200° C. and aratio of O₂ flow rate of 10%. Meanwhile for Sample 17, Sample 16, Sample15, and Sample 14 with a substrate temperature of 200° C. and ratios ofO₂ flow rate of 30%, 50%, 70%, and 100%, peaks indicating crystallinityare observed at 2θ in the vicinity of 31°.

Note that the peak indicating crystallinity at 2θ in the vicinity of 31°indicates a crystal part having a c-axis aligned in a direction parallelto a normal vector of a surface on which the IGZO film is formed, andtherefore indicates that the IGZO film is what is called in thisspecification a CAAC-OS film.

The above results show that the crystallinity of the IGZO film can becontrolled by changing the substrate temperature and the ratio of O₂flow rate, which are film formation conditions of the IGZO film.

Here, the substrate temperature, ratio of O₂ flow rate, Eg, and peak at2θ in the vicinity of 31° of Sample 1 to Sample 18 are shown in Table 1.

TABLE 1 Substrate Ratio of O₂ flow Eg Peak at 2θ in the temperature (°C.) rate (%) (eV) vicinity of 31° Sample 1 R.T. 100%  3.06 Not observedSample 2 R.T. 50% 3.06 Not observed Sample 3 R.T. 10% 3.06 Not observedSample 4 100° C. 100%  3.18 Observed Sample 5 100° C. 70% 3.11 ObservedSample 6 100° C. 50% 3.07 Not observed Sample 7 100° C. 30% 3.08 Notobserved Sample 8 100° C. 10% 3.06 Not observed Sample 9 170° C. 100% 3.21 Observed Sample 10 170° C. 70% 3.16 Observed Sample 11 170° C. 50%3.13 Observed Sample 12 170° C. 30% 3.10 Observed Sample 13 170° C. 10%3.07 Not observed Sample 14 200° C. 100%  3.21 Observed Sample 15 200°C. 70% 3.16 Observed Sample 16 200° C. 50% 3.14 Observed Sample 17 200°C. 30% 3.11 Observed Sample 18 200° C. 10% 3.08 Not observed

As shown in FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 4A and 4B, FIGS. 5Aand 5B, and Table 1, there is a correlation between Eg and the peak at2θ in the vicinity of 31° indicating crystallinity of the IGZO film: theIGZO films having Eg of more than 3.1 eV exhibit peaks indicatingcrystallinity at 2θ in the vicinity of 31°. In other words, it can besaid that an IGZO film having Eg of 3.1 eV or more is a CAAC-OS film.

EXAMPLE 2

In Example 2, transistors with the use of oxide semiconductor filmsincluding indium, gallium, and zinc (expressed as IGZO films below) weremade, and electrical characteristics of the transistors were evaluated.

A plan view of the transistors made in this example is illustrated inFIG. 1A, and a cross-sectional view of the transistors is illustrated inFIG. 1B. FIG. 1B corresponds to a cross-sectional view taken along lineX1-Y1 in FIG. 1A. Note that in FIG. 1A, some components of thetransistors (e.g., a gate insulating film 306) are not illustrated forsimplicity. The details of the transistors made in this example aredescribed below.

A glass substrate was used as a substrate 302. A titanium film(thickness: 100 nm) was used as a gate electrode 304. A film of stackedlayers of a silicon nitride film (thickness: 325 nm) and a siliconoxynitride film (thickness: 50 nm) was used as a gate insulating film306. Further, an IGZO film (thickness: 50 nm) formed by a sputteringmethod using a metal oxide target with an atomic ratio of In:Ga:Zn=1:1:1was used as an IGZO film 308. The IGZO film was, after the filmformation, subjected to heat treatment at 450° C. for 1 hour in the air.Then, a titanium film (thickness: 100 nm) was used as a source electrode310 a and a drain electrode 310 b. Further, a silicon oxynitride film(thickness: 265 nm) was used as an interlayer insulating film 312, andan acrylic resin (thickness: 2.3 μm) was used as a planarizationinsulating film 314.

The IGZO film 308 was formed under fixed conditions where the filmformation pressure was 0.3 Pa, and the film formation power was 11 kw(DC power source, the power density was 3.3 W/cm²), at various substratetemperatures and various O₂ flow rates (various ratios of O₂ flow rate)(Conditions A to Conditions D). The details of Conditions A toConditions D are described below.

(Conditions A)

-   Substrate temperature=100° C., Ar/O₂ flow rates=90/10 sccm (ratio of    O₂ flow rate=10%)

(Conditions B)

-   Substrate temperature=100° C., Ar/O₂ flow rates=50/50 sccm (ratio of    O₂ flow rate=50%)

(Conditions C)

-   Substrate temperature=200° C., Ar/O₂ flow rates=90/10 sccm (ratio of    O₂ flow rate=10%)

(Conditions D)

-   Substrate temperature=200° C., Ar/O₂ flow rates=50/50 sccm (ratio of    O₂ flow rate=50%)

A plurality of transistors each having the structure illustrated inFIGS. 1A and 1B was formed under each of the above-described ConditionsA to Conditions D. Note that six conditions of the channel length L (Lis indicated in FIG. 1A) were set for the transistors: 2 μm, 3 μm, 4 μm,12 μm, 46 μm, and 96 μm; and the channel widths W (W is indicated inFIG. 1A) of the transistors were fixed at 28 μm. Twelve transistors wereformed for each channel length L for each conditions, and electricalcharacteristics of the transistors were evaluated.

FIG. 6A shows results of the electrical characteristics of thetransistors with a channel length L of 2 μm which are formed underConditions A, FIG. 6B shows results of the electrical characteristics ofthe transistors with a channel length L of 3 μm which are formed underConditions A, and FIG. 6C shows results of the electricalcharacteristics of the transistors with a channel length of 4 μm whichare formed under Conditions A. FIG. 7A shows results of the electricalcharacteristics of the transistors with a channel length L of 12 μmwhich are formed under Conditions A, FIG. 7B shows results of theelectrical characteristics of the transistors with a channel length L of46 μm which are formed under Conditions A, and FIG. 7C shows results ofthe electrical characteristics of the transistors with a channel lengthL of 96 μm which are formed under Conditions A. Further, FIG. 8A showsresults of the electrical characteristics of the transistors with achannel length L of 2 μm which are formed under Conditions B, FIG. 8Bshows results of the electrical characteristics of the transistors witha channel length L of 3 μm which are formed under Conditions B, and FIG.8C shows results of the electrical characteristics of the transistorswith a channel length L of 4 μm which are formed under Conditions B.FIG. 9A shows results of the electrical characteristics of thetransistors with a channel length L of 12 μm which are formed underConditions B, FIG. 9B shows results of the electrical characteristics ofthe transistors with a channel length L of 46 μm which are formed underConditions B, and FIG. 9C shows results of the electricalcharacteristics of the transistors with a channel length L of 96 μmwhich are formed under Conditions B. Furthermore, FIG. 10A shows resultsof the electrical characteristics of the transistors with a channellength L of 2 μm which are formed under Conditions C, FIG. 10B showsresults of the electrical characteristics of the transistors with achannel length L of 3 μm which are formed under Conditions C, and FIG.10C shows results of the electrical characteristics of the transistorswith a channel length L of 4 μm which are formed under Conditions C.FIG. 11A shows results of the electrical characteristics of thetransistors with a channel length L of 12 μm which are formed underConditions C, FIG. 11B shows results of the electrical characteristicsof the transistors with a channel length L of 46 μm which are formedunder Conditions C, and FIG. 11C shows results of the electricalcharacteristics of the transistors with a channel length L of 96 μmwhich are formed under Conditions C. In addition, FIG. 12A shows resultsof the electrical characteristics of the transistors with a channellength L of 2 μm which are formed under Conditions D, FIG. 12B showsresults of the electrical characteristics of the transistors with achannel length L of 3 μm which are formed under Conditions D, and FIG.12C shows results of the electrical characteristics of the transistorswith a channel length L of 4 μm which are formed under Conditions D.FIG. 13A shows results of the electrical characteristics of thetransistors with a channel length L of 12 μm which are formed underConditions D, FIG. 13B shows results of the electrical characteristicsof the transistors with a channel length L of 46 μm which are formedunder Conditions D, and FIG. 13C shows results of the electricalcharacteristics of the transistors with a channel length L of 96 μmwhich are formed under Conditions D.

In each of FIG. 6A to FIG. 13C showing the electrical characteristics ofthe transistors, the horizontal axis represents the gate voltage (V)(expressed as Vg below), the vertical axis represents the drain current(A) (expressed as Id below), and data for the twelve transistors areshown at a time. Further, the voltage (V) between the source electrodeand the drain electrode (expressed as Vd below) was set at 10 V, and Vgwas applied from −15 V to 35 V at intervals of 0.5 V.

According to FIG. 6A to FIG. 13C, some of the transistors with a channellength L of 2 μm formed under Conditions A, Conditions B, and ConditionsC are brought into a conducting mode (normally on characteristics).Meanwhile the transistors with a channel length L of 2 μm manufacturedunder Conditions D are not brought into a conducting mode (normally oncharacteristics) and are thus found to have favorable transistorcharacteristics.

The results of Example 1 and Example 2 show that a higher ratio of O₂flow rate and a higher substrate temperature at the time of forming anIGZO film tend to suppress conduction failures.

Further, the results show that in order for a transistor using an IGZOfilm with a channel length of 2 μm to be free from conducting-modefailures, the substrate temperature is preferably 200° C. and the ratioof O₂ flow rate at the time of forming the IGZO film is preferably 50%.

As described above, a change in film formation conditions of the IGZOfilm 308 influences electrical characteristics of the transistor, andthe influence is particularly noticeable in the case of a transistorwith a short channel length L of 2 μm. The transistors formed underConditions D each have a peak indicating crystallinity at 2θ in thevicinity of 31°, which is described in Example 1, and thus each includea CAAC-OS film having a c-axis aligned in a direction parallel to anormal vector of a surface on which the IGZO film is formed.

Thus, it was revealed that the use of a CAAC-OS film in a transistor cansuppress conducting mode (normally on characteristics) of electricalcharacteristics in the case where the transistor has a short channellength. That is, a transistor with a short channel length can bemanufactured by using a CAAC-OS film.

This application is based on Japanese Patent Application serial no.2012-093303 filed with Japan Patent Office on Apr. 16, 2012, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: an oxide semiconductor layer,wherein a length of a channel formed in the oxide semiconductor layer ismore than or equal to 1 μm and less than or equal to 50 μm, and whereina diffraction intensity of the oxide semiconductor layer in X-raydiffraction measurement has a peak at a rotation angle 2θ of more thanor equal to 30° and less than or equal to 32°.
 2. The semiconductordevice according to claim 1 further comprising: a gate electrode; a gateinsulating layer over the gate electrode; and a source electrode and adrain electrode over the oxide semiconductor layer, wherein the oxidesemiconductor layer is over the gate insulating layer.
 3. Thesemiconductor device according to claim 1, wherein the length of thechannel is less than 5 μm.
 4. The semiconductor device according toclaim 1, wherein a band gap of the oxide semiconductor layer is morethan or equal to 3.1 eV.
 5. The semiconductor device according to claim1, wherein the oxide semiconductor layer includes at least one oxideselected from the group consisting of indium oxide, zinc oxide, galliumoxide, tin oxide and a combination thereof.
 6. The semiconductor deviceaccording to claim 1, wherein the oxide semiconductor layer is anIn—Ga—Zn-based oxide semiconductor layer.
 7. The semiconductor deviceaccording to claim 1, wherein the oxide semiconductor layer includes acrystal part, and wherein a c-axis of the crystal part is aligned in adirection substantially parallel to a normal vector of a surface onwhich the oxide semiconductor layer is formed.
 8. A semiconductor devicecomprising: an oxide semiconductor layer, wherein a length of a channelformed in the oxide semiconductor layer is more than or equal to 1 μmand less than or equal to 50 μm, wherein the oxide semiconductor layerincludes a crystal part, and wherein a c-axis of the crystal part isaligned in a direction substantially parallel to a normal vector of asurface on which the oxide semiconductor layer is formed.
 9. Thesemiconductor device according to claim 8 further comprising: a gateelectrode; a gate insulating layer over the gate electrode; and a sourceelectrode and a drain electrode over the oxide semiconductor layer,wherein the oxide semiconductor layer is over the gate insulating layer.10. The semiconductor device according to claim 8, wherein the length ofthe channel is less than 5 μm.
 11. The semiconductor device according toclaim 8, wherein the oxide semiconductor layer is an In—Ga—Zn-basedoxide semiconductor layer.
 12. The semiconductor device according toclaim 1 further comprising: a substrate; and a base insulating filmbetween the substrate and the oxide semiconductor layer.
 13. Thesemiconductor device according to claim 12, wherein the base insulatingfilm is one of a silicon oxide film, a silicon oxynitride film, asilicon nitride oxide film, a silicon nitride film and an aluminum oxidefilm.
 14. The semiconductor device according to claim 13, wherein thebase insulating film is the aluminum oxide film, and wherein a densityof the aluminum oxide film is 3.2 g/cm³ or more.
 15. The semiconductordevice according to claim 13, wherein the base insulating film is thealuminum oxide film, and wherein an amount of water (H₂O) released fromthe aluminum oxide film is less than or equal to 5×10¹⁵ atoms/cm³. 16.The semiconductor device according to claim 13, wherein the baseinsulating film is the aluminum oxide film, and wherein an amount ofhydrogen (H₂) released from the aluminum oxide film is less than orequal to 5×10¹⁵ atoms/cm³.